Code Division Multiple Access (CDMA) is a channel access technique that has found use in a variety of different applications. For example, some cellular phone services in the United States and elsewhere employ CDMA as a way to provide voice and data service for many customers at the same time in the same cell. In short, CDMA provides a way for multiple users to be multiplexed over the same physical channel.
CDMA has also found use in Satellite Positioning Systems (SPSs), such as the Global Positioning System (GPS). A particular CDMA technique is employed in SPSs, such as GPS, to allow the different positioning satellites to use the same two operating frequencies.
FIG. 1 is an illustration of a conventional autocorrelator 100 and a demodulator 150 for use in a CDMA receiver (not shown). The autocorrelator 100 includes a correlator module 101, a delay module 102, and a reference PRN module 103. A signal transmitted according to a CDMA technique is modulated with a pseudorandom (PRN) code. A receiver includes an autocorrelator, such as the autocorrelator 100, to pick a desired signal from a multitude of other signals using the PRN code of the desired signal. If a received signal matches the expected code of the desired signal, then the autocorrelation function (ACF) output by the correlator module 101 is high, and the system can use the signal. The autocorrelator 100 produces samples of ACFs, which it sends to a demodulator 150.
In addition to correlating signals based on codes, the autocorrelator 100 also correlates signals based on time. FIG. 2 is an illustration of an autocorrelation function (ACF) that represents an aspect of possible output of the autocorrelator 100. The x-axis of FIG. 2 is tau, which is a time delay relative to a carrier signal, and the y-axis is power. Tau can be applied by the delay module 102 of FIG. 1 during correlation. Specifically, the autocorrelator 100 can adjust a time delay, using the time delay module 102, when receiving a signal to discern a particular time delay where the received signal has its highest power. That particular delay can then be defined as tau equals zero in some systems. The signal shown in FIG. 2 is an autocorrelation function (ACF) of a well-defined signal that has an identifiable peak at tau equals zero.
FIG. 3 is another illustration of an autocorrelation function (ACF) that represents an aspect of a possible output of the autocorrelator 100. The ACFs 301 and 302 in FIG. 3 result from the correlation of a primary signal and a delayed signal, respectively. The ACFs 301 and 302 seen in FIG. 3 are often associated with a multipath phenomenon, where a transmitted signal reaches a receiver by more than one path (e.g., a line-of-sight path and a path due to reflection by a building). FIG. 3 is illustrative of a scenario wherein the time delay between the primary signal and delayed signal is large enough that the autocorrelator 100 can distinguish between the two signals. For CDMA-based SPS systems, delays of about two chips or more are usually resolvable. (In GPS systems one second equals 1.023 million chips.) In some CDMA systems, a rake receiver is used to combine distinguishable multipath signals in order to achieve a larger signal to noise ratio.
FIG. 4 is an illustration of an autocorrelation function (ACF) that represents an aspect of another possible output of the autocorrelator 100. FIG. 4 shows an ACF 400 that is a composite of two ACFS 401 and 402 from an indistinguishable multipath scenario. When contrasted with the ACF of FIG. 2, the ACF 400 has a larger width on the x-axis (i.e., is fatter) and has a rounded power peak around tau equals zero rather than the sawtooth-type shape of the ACF of FIG. 2, which shows a well-defined power peak centered at tau equals zero. The rounded shape is a consequence of the autocorrelator 100 failing to distinguish between a primary signal and a delayed signal, where the delay of the delayed signal relative to the primary signal is small. While shown as a bell-shape type curve in FIG. 4, ACFs resulting from indistinguishable multipath signals are not limited to such a shape. Usually, ACFs resulting from indistinguishable multipath signals show a wider range of power on the x-axis and can be irregularly shaped in some circumstances, causing difficulty for an application that looks for peak power.
For typical CDMA-based SPS systems, delays of about one chip or less are often indistinguishable. Indistinguishable multipaths are often undesirable because they cause a degradation of the information that can be gleaned from the signals. For instance, GPS systems typically identify peaks from ACFs to judge time of arrival of a signal from a Space Vehicle (SV). The fatness of an ACF with an indistinguishable multipath signal can lead to errors in judging signal time of arrival. In GPS, for example, a one-tenth of a microsecond time delay in a signal translates to about thirty meters so that accurate time estimation is usually important.
Currently, there is a need for detecting unresolvable multipaths quickly and efficiently from the output of the autocorrelator 100.